Crystalline polyamide, such as nylon 6 or nylon 66, has been widely used in fibers, films, or general-purpose engineering plastic materials, since it exhibits, for example, excellent moldability, mechanical properties, and chemical resistance. In recent years, in association with expansion of application fields of polyamide, increasing demand has arisen for transparent polyamide (i.e., a polymer having polyamide properties and transparency) to be used in, for example, films, eyeglass lenses, and covers of various devices.
There have been proposed various methods for producing transparent polyamide, including a method in which three or more monomers are copolymerized, and a method employing a diamine having a plurality of cyclohexane rings. For example, Patent Document 1 discloses a transparent thermoplastic polyamide produced, by a known method, through polycondensation between hexamethylenediamine and a mixture of isophthalic acid or a polyamide-formable functional derivative of isophthalic acid (70 to 85%) and terephthalic acid or a polyamide-formable functional derivative of terephthalic acid (30 to 15%). Patent Document 2 discloses a transparent polyamide resin formed of (A) a unit derived from a dicarboxylic acid and (B) a unit derived from a diamine, wherein the dicarboxylic acid of the unit (A) is formed of (A-1) a unit derived from a C6 to C22 branched saturated dicarboxylic acid (50 to 100 mol %) and (A-2) a unit derived from isophthalic acid (0 to 50 mol %); the diamine of the unit (B) is a diamine represented by a specific structural formula; and the ratio by mole of the unit (A) to the unit (B) is 100:95 to 100:105. Patent Document 3 discloses a colorless, transparent amorphous polyamide produced from a combination of a C14 to C22 alkyl-substituted alicyclic diamine and a C8 to C14 non-branched aliphatic dicarboxylic acid which may be partially replaced, up to 20 mol %, with an aromatic dicarboxylic acid, or a combination of a C8 to C14 non-branched aliphatic diamine and a C7 to C36 alicyclic dicarboxylic acid which may be partially replaced, up to 20 mol %, with an aromatic dicarboxylic acid. Patent Document 4 discloses a transparent thermoplastic polyamide produced from an alicyclic diamine, a lactam or the corresponding amino acid having at least seven carbon atoms, and terephthalic acid or an isophthalic acid-terephthalic acid mixture containing terephthalic acid in an amount of 50% or more.